1. Field of the Invention
This invention relates to functional deposited films, and a process for their formation and an apparatus for their continuous formation. More particularly, it relates to non-monocrystalline semiconductors formed by plasma-assisted CVD on a belt substrate (beltlike substrate, hereinafter referred to also as "strip substrate"), used in photoelectric transducers such as solar cells, and a process for their formation and an apparatus for their continuous formation.
2. Related Background Art
In recent years, the problems of environmental pollution have become serious. Power generation systems making use of photoelectric transducers that utilize energy of light such as sunlight do not cause the problems of disposal of radioactive contamination substances resulting from atomic power generation and the problems of environmental pollution due to power generation, e.g., the problem that may occur as the atmosphere becomes warmer because of thermal power generation. In addition, the sun shines everywhere on the earth and hence provides a less localized energy source, so that a relatively high power generation efficiency can be achieved without requiring any complicated and large-scale equipment. Thus, such systems have attracted attention as clean power generation systems that can cope with a future increase in the demand for power supply without causing any destruction of the earth's environment, and a variety of research and development studies have been made so that the systems can be put into practical use.
With regard to the power generation systems making use of solar cells, the solar cells used are required to have a sufficiently high photoelectric conversion efficiency, have a superior stability in their characteristics and be feasible for mass production, in order for them to be established as a system for meeting the demand for power supply.
In order to provide all the electric power necessary for typical homes, it is considered necessary to provide an amount of solar cells having an output of about 3 kW per household. Assuming that the solar cells have a photoelectric conversion efficiency of, e.g., about 10%, the solar cells must have an area of about 30 m.sup.2 in order to obtain the necessary output. Thus, in order to provide electric power necessary for, e.g., 100,000 homes, the solar cells must have an area of as large as 3,000,000 m.sup.2.
Under such circumstances, solar cells that can be produced by using a readily available gaseous starting material such as silane and decomposing it by glow discharge so that an amorphous silicon or the like semiconductor thin film is deposited on a relatively inexpensive substrate such as a glass or metal sheet have attracted attention because of their good mass productivity and their possibility of production at a low cost compared with solar cells produced using monocrystalline silicon or the like. Thus, various proposals have been made for their production process and apparatus.
In this connection, U.S. Pat. No. 4,400,409 discloses a continuous plasma-assisted CVD apparatus employing a roll-to-roll system. It discloses that this apparatus is provided with a plurality of glow discharge regions, where a sufficiently long, flexible belt substrate with a desired width is disposed along the path on which the substrate passes successively through the respective glow discharge regions in order, and the substrate is lengthwise continuously transported while desired-conductivity type semiconductor layers are formed by deposition in the respective glow discharge regions, so that large-area devices having semiconductor junctions can be continuously formed. Thus, this roll-to-roll system can be said to be feasible for mass production of large-area semiconductor elements.
Meanwhile, a plasma-assisted process making use of microwaves has recently attracted interest. Since the microwaves have a high frequency, the energy density can be made higher than in the case when conventional high-frequency waves with a radio frequency are used. Hence, the process is suited for generating plasma with a good efficiency and maintaining it.
For example, Japanese Patent Application Laid-Open No. 3-30419 discloses a deposited film forming method and apparatus of a roll-to-roll type making use of microwave plasma-assisted CVD. It discloses that generating plasma by the use of microwaves makes it possible to form deposited films even under a low pressure, so that not only any polymerization of active species that causes a lowering of film characteristics of deposited films can be prevented to obtain deposited films with a high quality, but also any powder of a polysilane or the like can be prevented from being generated in the plasma and also the film forming rate can be dramatically improved.
This publication also discloses an apparatus so constructed that an i-type semiconductor layer is formed by microwave plasma-assisted CVD and n-type and p-type semiconductor layers are formed by high-frequency plasma-assisted CVD so that photovoltaic elements having a pin structure can be continuously formed by the roll-to-roll system. In the case of photovoltaic elements comprising a non-monocrystalline semiconductor, a pin (or nip) layer structure is commonly employed. In such a layer structure, the i-type semiconductor layer is required to have a certain thickness in order to absorb incident light and on the other hand the n-type and p-type semiconductor layers are only required to have a very small thickness of about 1/10 of the i-type semiconductor layer. Hence, in the roll-to-roll system, the n-type and p-type semiconductor layers can be formed by high-frequency plasma-assisted CVD having a relatively low film forming rate. To form very thin semiconductor layers with good reproducibility by microwave plasma-assisted CVD having a very low film forming rate requires reasonable skill, and the very thin semiconductor layers can be more readily formed with good reproducibility by high-frequency plasma-assisted CVD having a relatively low film forming rate. In the roll-to-roll system, when microwave plasma-assisted CVD having a relatively high film forming rate is employed to form the i-type semiconductor layer, the transport speed of the belt substrate can be made higher than when high-frequency plasma-assisted CVD is employed. When the transport speed of the belt substrate is made higher, the time necessary for film formation is constant in the formation of the n-type and p-type semiconductor layers, and hence film-forming chambers must be made longer in proportion to the transport speed in the direction in which the belt substrate is transported. However, there is a limit when a thin and homogeneous non-monocrystalline semiconductor layer is formed in a long film-forming chamber in a large area with good reproducibility, tending to inevitably cause non-uniformity in layer thickness whereby the layer thickness is excessively smaller or larger than a prescribed layer thickness, or to cause irregularity in the characteristics such as conductivity. In particular, an impurity-doped layer (p-type or n-type) provided on the light-incident side of the i-type semiconductor layer must be made as thin as possible. It, however, is difficult to form a thin and homogeneous large-area impurity-doped layer in a long film-forming chamber by conventional high-frequency plasma-assisted CVD, causing non-uniformity or irregularity in the characteristics of photovoltaic elements thus fabricated.
In photovoltaic elements such as solar cells, unit modules of a photovoltaic element are often connected in series or in parallel to form a unit so that the desired current and voltage can be obtained. It is required for the unit modules to have less non-uniformity or irregularity in characteristics such as output voltage and output current between unit modules, and, at the stage of forming the unit modules, it is required to have a uniformity in the electrical characteristics of semiconductor multi-layer deposited films, which is the most important characterization factor. In order to simplify the step of assembling the modules, it should be possible to form semiconductor multi-layer deposited films having superior characteristics over a large area. This can bring about an improvement in mass productivity of photovoltaic elements such as solar cells and also a great decrease in production cost. In this regard, any conventional apparatus for continuously forming semiconductor multi-layer deposited films in which the i-type semiconductor layer is formed by microwave plasma-assisted CVD and the n-type and p-type semiconductor layers by high-frequency plasma-assisted CVD tend to cause non-uniformity or irregularity in the characteristics of semiconductor multi-layer deposited films for the photovoltaic elements fabricated, and have been problematic.
As a method for forming n-type and p-type semiconductor layers, ion implantation is also conventionally known in the art. The ion implantation method enables control of layer while the strength at which impurity ions are implanted is controlled by the accelerating voltage. However, ion implantation apparatuses with which impurity ions are implanted are commonly comprised of a system for generating ions, a system for extracting ions in the form of a beam, a system for causing the beam to scan, and so forth, resulting in a complicated structure and an expensive apparatus. Hence, they are not suited for forming the non-monocrystalline semiconductor photovoltaic elements with good productivity at a low cost, and have not been employed as means for forming impurity-doped layers.
Meanwhile, as a method for forming very shallow junctions required in ultra LSIs and so forth, plasma doping has recently attracted attention, and it enables incorporation of impurities by the use of a plasma of impurity gases, as reported in Ultra LSI Process Data Handbook (Science Forum, 1990 issue) and so forth. Lecture Draft Collections 30p-M-6 in 1988 35th Joint Lecture Meeting Concerned with Applied Physics also discloses that amorphous silicon films can be doped with impurities by plasma doping in which i-type amorphous silicon films are exposed to a high-frequency plasma of impurity gases. However, there has been no disclosure as to the application of such plasma doping to the formation of impurity-doped layers of photovoltaic elements such as solar cells. It has been unknown how plasma doping may be carried out to form good photovoltaic elements, when the photovoltaic elements in which i-type semiconductor layers are formed by microwave plasma-assisted CVD are fabricated.
In addition, amorphous silicon solar cells may experience a phenomenon in which the characteristics deteriorate as a result of irradiation with light (the Staebler-Wronski effect), which does not occur in crystalline solar cells. This is an important subject for achieving a cost decrease on the basis of efficiency improvement techniques and large-area production techniques and also for putting the solar cells into practical use so that they can be used as power supply means.
With regard to elucidation of the mechanism of this deterioration by light and a countermeasure therefor, a large number of approaches have been made from the viewpoint of semiconductor materials, e.g., to decrease impurities, from the viewpoint of devices, e.g., to employ a tandem device structure, and from the viewpoint of a treatment for the recovery of characteristics, e.g., to carry out annealing. In particular, the employment of the tandem device structure makes it possible to decrease the thickness of the i-type semiconductor layer, prevent the deterioration by light, and also achieve a higher efficiency by superposing solar cells having different band gaps, and hence in recent years has attracted attention.
Among tandem device structures, triple-layer tandem devices are advantageous over double-layer tandem devices in that incident light spectra can be utilized in a broader wavelength region, a higher photoelectric conversion efficiency can be obtained, and also a higher output voltage can be obtained. The triple-layer tandem devices, however, are comprised of a large number of layers (9 layers or more), and there has been a problem of how semiconductor multi-layer deposited films having this multi-layer construction can be formed with good reproducibility, at a high rate, and in a continuous manner.